Almost from the dawn of the industrial age, scientists were fascinated with the possibility of communication between remote points in coded, audio and visual formats. In France even as early as the late 1700's, elaborate semaphore systems enjoyed substantially widespread use. While such systems achieved their maximum readability during the darkness, and relied, to a large extent, on a subjective evaluation of a signal by the human eye in a sometimes noisy environment, the same represented a dynamic leap of progress over previously employed communications systems.
The invention of the telegraph by Morse in the early 1800's provided a means for rapid communication which effectively addressed virtually all the perceived limitations of semaphore communication. While the telegraph did require the installation of a telegraph wire hundreds and, ultimately, thousands of miles long, the telegraph insulated its users from dependence on good visibility conditions, fog, rain, atmospheric conditions and high levels of skylight due to natural and/or artificial causes.
Even before the invention of the telephone by Bell in 1876, it was recognized that electrical wires could be used to transmit video signals from a transmission point to a remote location. At least as early as the 1860's, French scientists proposed the possibility of scanning an object illuminated by candlelight using a Nipkow disk, reading the reflected light using a photoelectric device, and transmitting the signal over a wire to a remote point for viewing.
The weak point in that system (as well as in all modern video systems) was the display. Their proposed solution was to scan a sheet of paper mounted on a drum and impregnated with gunpowder with a high voltage ignition spark which burned in the image scanned by the Nipkow disk. While those familiar only with current state-of-the-art display technology might view such a technique as impractical, it was exactly this display technology which was employed by the great international news services during the first half of the 20th century to transmit photographs by wire.
Although this system had many inherent limitations, it had a number of virtues which no other widely employed display technology has succeeded in matching. For example, the system used very low power and produced very clear sharp images. Unlike liquid crystals, received pictures were visible over a wide angle of view. Unlike cathode ray tube images, images produced by this system enjoyed superb readability even under intense illumination. Still yet another advantage of this system was its extremely low cost.
Of course, such a system could only have limited application because of the exhaustion of the display member by a single frame of transmitted information.
While, during this early period in the history of video display technology, researchers working in the field may have entertained the possibility of a transient reflective mosaic as a video display, a transient controllable light source probably appeared to be a much greater possibility given the number of candidates which included, even at the turn of the century, the incandescent lamp, the neon lamp, and, of course, the cathode ray tube. The earliest employed "video" displays were signs, the most notable being so-called "neon" signs and incandescent bulb matrix arrays, such as those found on theater marquees.
However, with the rapid development of vacuum technology in the period surrounding World War I, the cathode ray tube became a practical solution, insofar as it relied upon plate, vacuum and grid technologies, all of which had been developed for other purposes.
Notwithstanding the limitations of the cathode ray tube, which included poor readability in sunlight, cumbersome size, excessively high voltage, the possibility of X-radiation, and so forth, researchers adopted what must now be considered a low-tech solution and proceeded instead to develop camera technology. Thus, even today, the cathode ray tube in a form substantially unchanged from its earliest embodiments remains the display standard, nearly a century after it was proposed.
When the time came to select a standard format for color television, a purely electronic display system was again selected. While some consideration was given to a rotating color filter wheel system developed by the Columbia Broadcasting System, the officials responsible for selection of a national color television standard were uncertain whether we would ever have the technology to reliably mechanically control a video display and thus opted in favor of what would also come to be recognized as a problematic approach, namely, the shadow mask cathode ray tube.
Nearly a half century later, however, the inherent limitations of the cathode ray tube have become painfully apparent. So-called "large screen" televisions can only be achieved by using small tubes and clumsy projection optics. Resulting pictures are of such low intensity that acceptable viewing can only be had in the dark. Stray light creates general deterioration in image resolution both by decreasing the signal-to-noise ratio in the display picture and reducing the chrominance content of the projected picture. The end result is a physically large, high voltage and high power system which produces a poor dim picture. Finally, there is a growing concern over CRT radiation output, above and beyond the X-band radiation problem which was substantially solved in the 1970's.
In an attempt to address these problems, manufacturers have turned to liquid crystal display technology. While such display technology may lend itself to relatively large flat displays which will operate at relatively low voltage, such displays are very expensive to manufacture and have poor visibility when viewed within the ideal angle of view and are substantially unreadable outside that angle of view. Likewise, color in LCD systems is of extremely poor quality.
A most promising candidate for the solution of the above problems is the LMC or light modulating capacitor. These devices come in a wide range of structures and include reflective as well as transmissive devices.
Generally, light modulating capacitors comprise at least one fixed electrode and an active electrode made of metallized plastic film. Modulation of light is achieved by physical displacement of the active electrode with respect to the fixed electrode, changing the reflective and/or transmission characteristics of the device. Actuation of the active electrode is accomplished by electrostatically attracting or repelling the variable electrode to a desired position. In the case of an active electrode made of metallized Mylar brand polyester film, the electrode is extremely light, may be prestressed to increase the range of configuration possibilities, and requires extremely low power and low voltage to operate effectively and quickly.
When I first proposed such a device in the early 1970's, the active electrode generally had the shape of a flapper which was electrostatically driven from one position to another, typically in a two color grove having a V shaped cross-section, much like a pair of differently colored pages in a half-opened book. Because the flapper is highly reflective, when it is in a first position, it reflects the color of the inside of the groove on the side of the groove opposite that on which it is resting. Thus, with each side of the groove is given a different color, the groove appears be to completely the color of the side opposite the active electrode. Because this could be a reflective device, it operated well in ambient light and with only the smallest consumption of electricity insofar as the light modulating capacitor would only pass enough current to charge its internal capacitance.
I have previously proposed the possibility of a prestressed metallized Mylar electrode which, in its relaxed state comprised a tightly coiled active electrode which would be electrostatically unrolled over a flat panel, thus changing the color of the flat panel to the color of the active electrode with the device configured as a light reflecting capacitor. I have also suggested the possibility of a light transmitting window where the device might be backlit and the active electrode used to control the transmission of light through the device.
Similarly, I have proposed the possibility of a large matrix of light modulating capacitors being manufactured in a mass production operation and comprising a single multi-pixel module. In this system, the pixel took the configuration of a V profile flapper-type device.
My prior U.S. Pat. No. 3,989,357 discloses various light-modulating movable electrodes cooperative with a reflective backdrop to provide a reflectance zone whose reflectance properties can be controlled by appropriate selection of the reflectances of a viewer-facing electrode surface and of the backdrop. In this manner, simple messages, message elements or color pixels can be displayed by providing different areas of different reflectances in the reflectance zone. The position, or extent of excursion, of the movable element determines the relative sizes of the different areas. The reflectance of the movable element can be selected by giving it a highly reflective, viewer-facing surface (column 5, lines 25-30). The backdrop is generally opaque, being constituted in many embodiments either by an insulator, or if the insulator is transparent, by a fixed conductor which is generally metallic. Though useful for simple indications and for high contrast monochrome renditions of messages or images, the capabilities of the combination of reflectances that can be produced by this system is inadequate to meet more sophisticated requirements, for example, the demands of a full-color display.
To overcome this problem, one embodiment of my U.S. Pat. No. 3,989,357 (FIGS. 13-14, column 4, lines 44-68), proposes a display incorporating each of three primary colors juxtaposed to be visually mixed (presumably by a distant viewer). The arrangement shown employs colored, metal-foil, movable electrodes arranged in parallel layers on what are presumably opaque fixed-electrode-insulator assemblies. The colored tips of the movable electrodes are exposed to view but are not in a common well-defined reflectance zone, and are not presented perpendicularly to, or nearly perpendicularly to a light path to the observer. Indeed, the construction of the embodiment of FIGS. 13-14 in addition to providing only a weak visual appearance, is too cumbersome to be readily employed as a pixel construction in an array comprising hundreds of pixels.
While providing the possibility of some limited-range color blends, this device is still not adequate for the needs of a full-range, or multi-color display, especially a pixellated, continually variable display. The teaching of my prior patent provides inadequate brightness and insufficient variability of reflectance within a well defined pixel area transverse to a light path to a viewer.
The embodiment of FIG. 5 of my U.S. Pat. No. 3,989,357 (column 5, line 60 to column 6, line 10) employs a transparent insulator together with a somewhat transparent fixed electrode (see column 4, lines 61-65) in a transmissive mode in which light from a lamp is gated by a movable electrode. The discussion of the transparency of aluminum, which has to be balanced against its conductivity, suggests such a transparent fixed electrode would not be suitable for use in a reflective mode in which an incident light beam, passing through a transparent insulator and transparent or translucent fixed electrode, is reflected back to an observer from a backdrop of selected reflectance.
My U.S. Pat. No. 4,266,339 discloses various methods for manufacturing rolling electrodes for electrostatic devices. Disclosed is the fact that leaves of metallized plastic film, such as MYLAR (trademark DuPont) will curl in an oven to provide prestressed movable electrode elements. The disclosed methods include (column 3, lines 34 et seq.) cutting or punching U-shaped incisions through a metallized sheet, masking areas between the U-shaped cutouts and heating the punched, masked sheet in an oven, then cooling, to cause the unmasked pieces of sheet to curl up into tight rolls. Some drawbacks to this method arise from the need for a mask and difficulties in controlling the cutouts before they roll up.